Rebar rust is terribly damaging to reinforced concrete slabs and other structures because rust has two to four times the volume of clean steel but only a tiny fraction of steel's mechanical strength. The expanding rust breaks the bond between the steel rebar and the concrete and pushes outward, causing chips and chunks of concrete to split off the exterior. The breakdown of the exterior concrete eventually exposes the rebar. This allows more corrosive moisture to penetrate deep into the concrete, further rusting and weakening the steel and cracking the concrete until the slab crumbles or the structure collapses from the deterioration.
Preventing Rebar Rust
The main way to preserving reinforced concrete is prevention. Once the deterioration takes hold, little can be done to stop it. Rebar rust prevention requires a concrete mix with a high proportion of cement and aggregates to water. Concrete that is too wet will be more porous, which promotes rebar rust. The concrete also needs to have the correct volume of air bubbles that act to trap corrosive moisture. Concrete also needs to be properly finished to reduce surface cracking that allows moisture to penetrate.
You also need to have a thick concrete layer on top of the rebar to keep rust away from the steel. The NRMCA website says the rebar should be at least 1.5 in. below the surface in normal environments, and 2 to 2.5 in. below the surface in harsh salt-laden environments. Concrete should be poured over the rebar framework in warm temperatures (about 70 degrees Fahrenheit) and cured for at least a week before you apply loads to it.
Additives Can Help
Rebar rusting can be retarded with use of concrete additives that make the concrete less porous. Common additives, says the NRMCA, include silica, calcium nitrite, fly ash and blast-furnace slag. Other approaches to protect the rebar include applying an epoxy coating on the steel rods before they are installed, active cathode protection that uses ions to retard rebar rust and concrete sealants and water repellants.